On Sun, 16 Oct 2005 14:58:52 GMT, Cecil Moore wrote:
How long must the 50 ohm coax connected to the SWR meter be
for the SWR meter to report a valid SWR on that coax? The
answer obviously cannot be zero length.
Already back peddling I see.

"Cecil Moore" wrote in message
t...
Dave wrote:
"Cecil Moore" wrote:
And if you will remember, the original question didn't involve the
SWR meter at all. The question was: Are my 2-foot sections of RG-400
connected to my SWR meter long enough to ensure that the SWR meter
reading is valid for the coax? Remember that argument?

and the answer is .2"

Not for 6" diameter hard line. :-)

yeah, in your dreams your hardline as 6" in diameter! its probably not even
6" long!

by the time you put enough adapters on it to get it down to your so-239 or n
connector on your cheap swr meter it will be.

Richard Clark wrote:
Cecil Moore wrote:
Myth YES.

Third-hand knowledge, Junk or Treasure?

Actually, it doesn't rise to the definition of a myth.
It wasn't a popular belief. :-)
--
73, Cecil http://www.qsl.net/w5dxp

On Sun, 16 Oct 2005 14:58:52 GMT, Cecil Moore wrote:

Walter Maxwell wrote:
The line impedance of either the input or load transmission line is irrelevant
to the basis for the Zo of the device, therefore a length of line is unnecessary
to establish a Zo environment, the current sample and voltage sample establish
it..

Walt, there's one thing I don't understand about what Dave reported.
As I understand it, these were the test setups.

TDR---50 ohm coax---MFJ---50 ohm coax---50 ohm load

The TDR doesn't see the MFJ wattmeter.

The TDR doesn't see the wattmeter because there is no discontinuity.

TDR---75 ohm coax---MFJ---75 ohm coax---75 ohm load

The TDR sees the MFJ wattmeter.

It seems to me the amount of the discontinuity the wattmeter calibrated to 50
ohms the TDR sees would be determined by the tightness of coupling of the
sampling inductance and capacitance to the main line.

If the MFJ in the second setup were recalibrated for 75 ohms,
would the TDR not see it?

I believe the TDR would not see it.

************************************************* ***************

The threads for the past few days have all diverged from the
original question which was:

How long must the 50 ohm coax connected to the SWR meter be
for the SWR meter to report a valid SWR on that coax? The
answer obviously cannot be zero length.

Cecil, as I explained earlier, if we're considering the Bruene type SWR
indicator, the setting of the sampling cap in relation to the inductive sampling
determines the measurement reference, not the length or Zo of the coax. So I
repeat, with this SWR indicator zero coax length is sufficient. It will produce
the same answer with a lumped impedance as the load as it will with the same
impedance appearing as the input impedance of a coax.

Walt, W2DU

Owen Duffy wrote:
The myth: Measurements with a Bird 43 of the conditions on the
Thruline section are invalid unless it has some minimum length of 50
ohm line on both sides of itself.

Myth or not, YES or NO?

Actually, upon closer examination, the definition of "myth"
proves the above is NOT a myth. It is not a traditional story.
It is not a parable allegory. It is not a popular belief.

So, no Owen, it is not a myth.
--
73, Cecil http://www.qsl.net/w5dxp

On Sun, 16 Oct 2005 10:43:18 -0400, Walter Maxwell
wrote:

We are discussing the Bruene version of the device in which the inductance
derives a sample of the current and C1 derives a sample of the voltage. When C1
is adjusted to obtain the correct voltage for the voltage-current ratio to equal
Zo at that point, such as 50 ohms, the device provides the correct readings when
the line impedance on the load side is Zo. The line impedance Zo on the input
side is irrelevant.

Hi Walt,

You might force what appears to be SWR meter action, but what you
describe above is not sufficient to offer the original accuracy; and
at that point why bother putting a screwdriver in and twisting things?
To accomplish the purpose being discussed takes TWO changes (of the
THREE variables of design) as I have already discussed.

Except under the most fortuitous of circumstances, there is nothing
inside the meter that even closely approaches Z0 - or has to.

As an aside to your comments and directed to the novitiates, any
number of Bruene variants can be found where those variants offer less
than optimal performance. However, quality equipment offers that the
line transiting between input and output connectors and thru the
sensing core IS Z0. This consideration takes so little effort, that
those designs that do not are probably suffering other short-cuts away
from intelligence. This opprobrium goes equally for arguments that
presume there is no line either.

73's
Richard Clark, KB7QHC

Walter Maxwell wrote:

Cecil Moore wrote:
How long must the 50 ohm coax connected to the SWR meter be
for the SWR meter to report a valid SWR on that coax? The
answer obviously cannot be zero length.

Cecil, as I explained earlier, if we're considering the Bruene type SWR
indicator, the setting of the sampling cap in relation to the inductive sampling
determines the measurement reference, not the length or Zo of the coax. So I
repeat, with this SWR indicator zero coax length is sufficient. It will produce
the same answer with a lumped impedance as the load as it will with the same
impedance appearing as the input impedance of a coax.

But Walt, there no SWR possible on a zero length line. I think it
was Reg who said that first. I can now see how the initial confusion
arose and why the thread drifted.

Source---75 ohm line---SWR meter---load

The SWR meter will not report the actual SWR on the external 75 ohm line.

Source--75 ohm line--50 ohm line--SWR meter--load

The SWR meter will report the actual SWR on the external 50 ohm coax if
the external 50 ohm coax is _______ long.

Turns out the answer is a lot smaller than I previously thought.
The answer is less than one inch for the most popular 50 ohm coax's.

I didn't mean for the original question to be a trick question but it
apparently turned out that way.
--
73, Cecil http://www.qsl.net/w5dxp

On Sun, 16 Oct 2005 17:43:40 GMT, Cecil Moore wrote:
Cecil Moore wrote:
Myth YES.
Third-hand knowledge, Junk or Treasure?
Actually, it doesn't rise to the definition of a myth.
It wasn't a popular belief. :-)
Junk³

On Sun, 16 Oct 2005 14:44:09 GMT, Cecil Moore wrote:
Myth YES.
On Sun, 16 Oct 2005 17:49:44 GMT, Cecil Moore wrote:
not a myth.
must be a democrat from Massachusetts running for president.

On Sun, 16 Oct 2005 17:49:44 GMT, Cecil Moore wrote:

Owen Duffy wrote:
The myth: Measurements with a Bird 43 of the conditions on the
Thruline section are invalid unless it has some minimum length of 50
ohm line on both sides of itself.

Myth or not, YES or NO?

Actually, upon closer examination, the definition of "myth"
proves the above is NOT a myth. It is not a traditional story.
It is not a parable allegory. It is not a popular belief.

So, no Owen, it is not a myth.

Cecil, this is the opposite to your stated YES opinion just three
hours ago.

I am beginning to form the view that your opinion is just the opposite
of that expressed in the last posting you read, that you will argue
for arguments sake. When I look for consistency in you postings, it is
that they are consistently arguments against other propositions, even
if it means you have to change your own position to remain opposed. To
be kind, the devil's advocate.

Owen
--